r/vmagent: Rework as container deployment

Like the _blackbox-exporter_ role, the _vmagent_ role now deploys
`vmagent` as a container.  This simplifies the process considerably,
eliminating the download/transfer step.

While refactoring this role, I also changed how the trusted CA
certificates are handled.  Rather than copy files, the role now expects
a `vmagent_ca_certs` variable.  This variable is a mapping of
certificate name (file name without extension) to PEM contents.  This
allows certificates to be defined using normal host/group variables.

roles/vmagent/handlers/main.yml

@@ -8,6 +8,7 @@
     state: restarted
 
 - name: reload vmagent
-  uri:
-    url: http://[::1]:8429/-/reload
+  service:
+    name: vmagent
+    state: reloaded
   changed_when: true